Pressure sensing on rigid sails

ABSTRACT

Pressure tapping for rigid sails, the tappings (10, 11, 12, 15, 16) being located to either provide a stall warning or to enable tuning of the sailset to the desired operating angles. A particular flow-state sensitive stall warning device is also described.

This invention relates to rigid sails and in particular to monitoringwind pressure and/or flow conditions in the vicinity of such sails.

The type of sailset to which this invention is applicable generallycomprises one or more rigid aerofoils, each of which is constructed inthe form of a wing (as of an aircraft) although the fabric from which itis made may be different to the fabric of aircraft wings. Usually thecross-section of the aerofoil will be symmetrical but asymmetriccross-sections are not excluded. The aerofoils are mounted to rotateabout upright axes which may pass through the aerofoil or be remote fromit and connected for example by a boom. Trimming of the sailset isachieved by rotating one or more of the aerofoils about its axis.Sailsets of this type are described in the published European PatentApplications 61291 and 77205.

During sailing there is a constant need to select the angle of attackfor the aerofoils that gives the required thrust, which may be a varyingpercentage of the maximum available thrust, and to protect the sailsetand vessel from the excessive forces of high winds. Additionally it maybe desired to incorporate specialised systems that modulate the trimmingaction, for example to maintain a constant angle of heel while sailingor, when the vessel is moored, to minimise warp tension, roll, send etc.

In order to provide an automatic or semi automatic system that respondsto the prevailing conditions and the input direction and thrust demands,it is desirable to monitor the precise wind and flow conditions in thevicinity of the aerofoil.

Accordingly the invention provides a sailset comprising at least onerigid aerofoil and including at least one pair of pressure tappingsarranged to give an indication of the state of airflow in the vicinityof the aerofoil surface.

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIGS. 1a-1c illustrate schematically the flow conditions around anaerofoil sailset at the onset of stalling;

FIG. 2 shows an aerofoil section with pressure tapping points inaccordance with the invention,

FIG. 3 shows schematically in section, an aerofoil sailset with analternative arrangement of pressure tapping points in accordance withthe invention, and

FIG. 4 shows schematically a flow sensing and stall warning device inaccordance with the invention.

For the avoidance of doubt, the terms lift, drag and thrust have thefollowing relationship. Lift is the crosswind force, drag is thedownwind force and thrust is the vector sum of lift and drag.

Referring to FIG. 1, an aerofoil sailset is represented in sectionalview. The sailset consists of a leading sail element 1 that willgenerally be pivoted about an upright axis passing through the aerofoil,and a trailing sail element 2 that will generally be pivoted about anupright axis that is carried by the leading sail element 1 and passesthrough the leading sail element towards its trailing edge. The trailingelement 2 is attached to its axis by booms which permit the trailingelement to be swung from one side of the leading element 1 to the otherin mirror image configurations. A slat or flap may be attached to thetrailing edge of the leading element 1 in order to enhance theaerodynamic slot configuration formed between the two elements. Theelements 1 and 2 constitute the principal sail of the sailset and thewhole assembly may be rotated by the action of a tail vane (not shown).

In FIG. 1a the wind, indicated by the arrow, is incident to the leadingelement 1 at an angle of -10° and the air flow around the aerofoils issmooth and `attached` with the exception of a small separated wake 3a atthe trailing edge of the trailing section 2, on its low pressure side.The presence to a significant degree of this separated wake signals theonset of stalling and as the degree of stalling increases the separatedwake moves further along the aerofoil towards the leading edge (i.e. theseparation occurs earlier in the airflow) as represented in FIGS. 1b and1c, respectively, by references 3b and 3c, where the wind is incident at0° and +10°.

For a single aerofoil the onset of stalling occurs in the same way: at acritical point the airflow becomes separated, first at the trailing edgeon the low pressure side and then progressively along the low pressureside towards the leading edge. At the stall a point is reached at whichthe aerofoil no longer provides useful thrust.

Except for particular circumstances such as running downwind, it isgenerally the aim to avoid stalling the sailset. For a particularsailset and control system the preferred operating angles of attackdepend on the positions of the aerofoils in which maximum lift andmaximum lift:drag ratio occur. The lift maxima may occur at positions inwhich the sailset is close to stalling, and as the wind can shiftrapidly, perhaps by 40° per second, and the control system for rotatingthe aerofoils has a finite response time, the preferred operatingpositions are chosen to be a safe margin from the stalling positions,for example 4° off the position where the respective maxima occurs.

In the present invention pressure tapping points are located atpositions where, by adjusting the angle of attack so that a differencevalue for the pressure tappings is zero, the sailset is located in thepreferred operating position.

In one embodiment of the invention, with the sailset arrangement asgenerally illustrated in FIG. 3 pressure tapping points 10, 11 and 12are provided around the leading edge of the leading section, signalsfrom the three pressure tappings being supplied to a computing meansthat adjusts the angle of the sailset in accordance with the criteriademanded. For optimum thrust adjustment is made until the followingrelationship is satisfied

    2P.sub.10 -P.sub.11 -P.sub.12 =0

where

P₁₀ is the pressure at tapping 10

P₁₁ is the pressure at tapping 11

P₁₂ is the pressure at tapping 12

Tapping 10 is located on the centre line of the leading aerofoilelement. The positions of tappings 11 and 12 determine the precise angleof incidence of the wind to which the sailset is `tuned`. It has beendetermined that a preferred location for tappings 11 and 12 is at adistance of 5% of the total chord around the perimeter from the leadingedge and tapping 10. With these pressure tapping locations the aboverelationship is satisfied when the angle of incidence to the wind iszero.

By placing the tappings closer to the leading edge the relationship issatisfied when the angle of incidence is negative, for example if thetappings are located at a distance around the perimeter of 21/2% of thetotal chord then the angle of incidence for which the relationship issatisfied -1°. This will be further from the stalling position than whenthe tappings are at the 5% points, and also the tuning is less sensitiveas the rate of change of the value of the relationship close to the zeropoint is lower.

If the tappings are located further away from the leading edge, forexample at the 10% points, then the relationship is satisfied by apositive angle of incidence (11/2° for the 10% points) and the sailsetis operating closer to the stalling position. The problem with tappingsat these latter points is that the value of the relationship and hencethe sensitivity may suffer due to the pressure versus angle of incidencecurves exhibiting a hysteresis between moving into and moving out of thestalled state.

In the event of a deviation of the value of 2P₁₀ -P₁₁ -P₁₂ from zero,the control system or computing means will rotate the sailset until thezero value is regained. On a given tack a positive deviation willindicate that rotation in one direction is required, a negativedeviation indicating the reverse rotation. The sense of rotationrequired for a given deviation changes with a change in tack, andtherefore the computing means is also supplied with informationregarding which tack is being sailed. This could be incorporated by wayof a switching system linked to the movement of the trailing aerofoilelement or taken from a wind vane, or various other means.

In order to provide optimum lift:drag ratio the relationship that is tobe satisfied is P₁₁ -P₁₂ =0. While the tappings at the 5% point a zerois established when the angle of incidence of the wind is -16.5°. Whenthe tack is changed the sign of the difference changes and so, forexample, a positive is clockwise, negative is anti-clockwise conventionmay be adopted. (Positive is clockwise means that tapping 11 is on thestarboard side). If a higher lift:drag ratio is desired the tappings maybe placed further around the perimeter from the leading edge.

In storm conditions when it is desired to protect the sailset fromexcessive forces, the trailing section is aligned with the leadingsection and the sailset is maintained at zero angle of incidence. Forthis purpose the difference between pressure tappings 11 and 12 is alsoheld at zero.

In all cases the figures given are approximate, and for a given sailsetthe precise location of the tappings should be determinedexperimentally, for example by wind tunnel experiments.

Referring now to FIG. 2, a single aerofoil section is shown on which thepoints marked 10, 11, 12, 13, 14, 15 and 16 represent pressure tappingpoints according to the invention. All the pressure tapping pointsmarked need not be incorporated in every embodiment of the invention, orin some embodiments there may be tappings all along one or both sides ofthe aerofoil. Symmetrical arrangements of tappings are generallypreferred as in most circumstances the aerofoils are symmetrical and itis desired that the sailset function similarly in each of the mirrorimage configurations, that is function similarly on both port andstarboard tack.

As illustrated the tappings are located symmetrically in pairs onopposite sides of the aerofoil, with the exception of a lone tapping onthe centre line of the leading edge. Each of the tappings is arranged togive a signal, such as via valves and pressure transducers, to acomputing means that then determines the optimum sailing conditionsand/or alerts for stalling conditions.

The pressures at the tappings may be compared in either or both of thefollowing ways in order to give an indication of the state of flow.

Firstly the difference between a pair of tappings on the low pressureside of the aerofoil may be monitored. That is the difference betweentappings 11 and 13, or between tappings 12 and 14 depending on the tack.When smooth flow exists the pressure difference is small, but at theonset of stall the pressure difference increases as the separated wakehas a lower velocity and produces a higher pressure (which leads toreduced lift). When monitoring for stalling conditions in this way thesecond tapping of the pair (or a further pair) should be placed at thepoint on the aerofoil at which the extent of the separation of theairflow is tolerable but a warning is desired. Of course by using manypairs of tappings the exact progression of the separated airflow may beascertained.

Alternatively the pressure difference between tappings on opposite sidesof the aerofoil may be monitored. In this instance each one of the pairs11 and 12, 13 and 14 and 15 and 16 are monitored. When smooth airflowgiving thrust exists, there is a pressure difference between the highand low pressure side of the aerofoil. In stalling conditions thepressure on the low pressure side increases due to the separation of theairflow and thus the pressure difference decreases. In the instance oftappings located as in FIG. 2, the pressure difference will fall firstbetween pair 15 and 16, then between pair 13 and 14 and finally betweenpair 11 and 12. Clearly by putting further pairs between the pair 15 and16 and the pair 13 and 14 a closer monitoring of the onset of stallwould be obtained.

In practice it is not necessary to have pressure tappings all along theaerofoils: a set of three pressure tappings around the leading edge issufficient for alignment purposes and/or a pair close to the trailingedge as a stall warning system. FIG. 3 illustrates this combinationadapted for a sailset with leading and trailing elements, the stallwarning tappings being located on the trailing element. Due to thevariation in conditions at different heights it is preferable to averagethe signals (in the computing means) from several sets of tappingsspaced apart in the spanwise (upright) direction.

For protection against the ingress of weather the tappings may be closedby an impermeable membrane of low modulus of flexure to transmitpressure signals to the interior so nearly as possible unmodified by thetension in the membrane. To protect against ice local heating or lowfreezing point spray devices, or other suitable means are included.

A limitation that can occur using a system with, say, three pressuretappings at a given level at or near the leading edge of an aerofoil isthat small errors in location of the tappings causes a significant errorin the angular adjustment of the aerofoil. Also gradual deterioration ofthe smoothness and symmetry of the aerofoils due to wear would not beaccounted for by the control system which simply relies upon localpressure information.

In order to remove this limitation, in a preferred embodiment of theinvention the pressure tappings are modified to provide a device thatmonitors the flow state for varying wind speeds. Referring to FIG. 4,the device comprises a box 17 recessed in a cavity in the aerofoilstructure, the outer surface of the box comprising a flap 18 that ishinged or otherwise pivoted at 19 along the edge closest to the leadingedge of the aerofoil. The flap 19 is arranged so that its hinged edge isgenerally transverse to the direction of airflow along the surface ofthe aerofoil, and when in the closed position the flap lies flush withthe surface of the aerofoil. The edges of the flap 18 are sealed to thesides of the box by a flexible diaphragm 20. The flap is lightly biased,such as by spring loading outwards. The interior of the box is suppliedwith a pressure representative of the external wind speed by means of apitot tube 21, the probe end 22 of the tube being mounted at or in frontof the leading edge of the aerofoil. Thus the combination of the pitotpressure and the biassing tends to push the flap 18 outwards. Whensmooth flow over the surface of the aerofoil exists the kinetic energyof that flow prevents the flap from opening and opposes the outwardbiassing and pitot pressure. In stalling conditions the kinetic energyis small, or even reversed and the pressure is insufficient to preventthe flap from moving outward, to the extent permitted by the diaphragm20, and separating contact points 23, which initiates a stall warningindicator. (Of course the contacts could be arranged so as to makecontact as the flap moves outward).

This device has the advantage that it is triggered by a change in flowstate away from smooth high kinetic energy flow, and operatesindependently of wind speed, as the pitot probe transmits a compensatingpressure. Depending on the positions of these devices, they aretriggered at various lift:drag ratios, those towards the trailing edgebeing influenced by the stalling wake before those located closer to theleading edge.

In another embodiment of the invention a pair, or spanwise separatedpairs, of pressure tappings located approximately at the maximumdiameter of the aerofoil (i.e. the maximum thickness perpendicular tothe chord) are monitored for pressure difference ΔP. The liftcoefficient C_(L) is the integral of the pressure coefficients C_(p)over the whole aerofoil surface.

Pressure on aerofoil=C_(p) ×1/2ρv² where, C_(p) is the coefficient ofpressure, v is the stream velocity, and ρ is the density of fluid 1/2ρv²is called the kinetic head (which may be measured by a pitot-staticassembly).

Thus pressure/1/2ρv² =C_(p)

This relationship gives C_(p) from two pressure tappings and apitot-static assembly.

Alternatively an anemometer could be used to calculate the value of thekinetic head, the anemometer yielding an output proportional to thevelocity, and ρ is a constant.

Having the capacity to establish C_(p) at any particular point enablesthe value of C_(p) to be plotted for all angles of the sailset. Thus, ina sailset as illustrated in FIG. 3, the sailset may be set to rotate,say at an angular rate θ, thus causing the leading section to rotate andthe angle of incidence to change. By monitoring the value of C_(p), acurve of C_(p) versus angle of incidence may be plotted for the currentprevailing conditions. At the stalling point the rate of increase ofC_(p) with respect to θ (or other monitored variable related to theangle of incidence) will fall, eventually becoming negative. Havingestablished the current C_(p) versus θ curve and established the currentC_(p) maximum the sailset may then be set at the desired operating C_(p)which will be a safe level ΔC_(p) below the maximum C_(p) value.

Periodically the C_(p) versus θ curve will be redrawn and the sailsetadjusted accordingly. It is envisaged that the sailset may perform a `θsweep` excursion every three to five minutes. Typically it may beexpected to find C_(p) maximum at about +5° to +10°, and lift:dragmaximum at about -15° to -20°.

In all embodiments it may be advantageous to utilise a wind vaneupstream of the sailset to provide the control system with approximatevalues for the angle of incidence, the pressure tappings being used forfine adjustment.

I claim:
 1. A sailset comprising a leading rigid upright aerofoilmounted for rotation about an upright axis and a trailing rigid uprightaerofoil pivotable about an upright axis to adopt positions camberedwith respect to the leading element, a pressure tapping located on thecentre line of the leading edge of the leading aerofoil, a pair ofpressure tappings located symmetrically on opposite sides of the leadingedge of the leading aerofoil and control means for rotating the sailsetresponsive to a difference in the difference values between the pressuresensed at the central tapping and that at each of the pair of tappings.2. A sailset according to claim 1 in which the control means rotates thesailset to maintain a zero difference in said difference in thedifference value.
 3. A sailset according to claim 1 in which the controlsystem is settable to monitor the difference in the pressures at saidpair of tappings and to rotate the sailset to maintain a zero differencein those pressures.
 4. A sailset according to claim 1 in which the pairof tappings are located at a distance around the leading edge from thecentre line of up to 10% of the total chord.
 5. A sailset according toclaim 4 in which the pair of tappings are located at a distance aroundthe leading edge from the centre line of 5% of the total chord.
 6. Asailset according to claim 1 including a plurality of pairs of tappingslocated symmetrically on opposite sides of the leading edge of theleading aerofoil and the control means is settable to respond to valuesderived from a selected pair of said plurality of pairs.
 7. A sailsetaccording to claim 1 comprising an additional pair of pressure tappingslocated towards the trailing edge of the trailing section and means formonitoring said additional pair of pressure tappings to provide a stallwarning.
 8. A sailset according to claim 7 in which said additional pairof pressure tappings are located on opposite sides of the trailingaerofoil.
 9. A sailset according to claim 1 further comprising means forsweeping the sailset through a range of angles and monitoring pressuredifferences between tappings for the swept range of angles.
 10. Asailset according to claim 1 comprising a further pair of pressuretappings located at substantially the maximum diameter of the leadingaerofoil, and means for sweeping the sailset through a range of anglesto plot a value dependent on the pressure difference of said furtherpair of tappings for the swept range of angles.
 11. A sailset comprisinga leading rigid upright aerofoil mounted for rotation about an uprightaxis and a trailing rigid upright aerofoil pivotable about an uprightaxis to adopt positions cambered with respect to the leading element, atleast one pair of pressure tappings located symmetrically on oppositesides of the leading element, control means for rotating the sailset tomaintain predetermined difference values between the pressures at thetappings and means for sweeping the sailset through a range of angles toplot a value dependent on the pressure difference of a particular pairof pressure tappings.
 12. A sailset according to claim 11 in which saidparticular pair of pressure tappings are located at the maximum diameterof the leading section.